In a typical paper machine, a pulp furnish consisting of fibers and filler slurry is jetted from a headbox onto a moving paper forming surface, usually a wire. The wire is an endless loop; the top carrying pulp furnish is usually called the forming section. Underneath the forming section are many stationary drainage elements. They assist drainage in different ways depending on their design and position on the machine.
First, the wire travels across a series of hydrofoils (or table rolls in early models of Fourdrinier machine). White water drains from the pulp in response to the gravity and to the pulsation forces generated by these drainage elements. (The drainage in this period is therefore referred to as the pulsation/gravity drainage). Furnish consistency increases gradually and dewatering becomes more difficult as the wire travels farther down the table. Vacuum assisted hydrofoils are used to sustain higher drainage, and then high-vacuum flat boxes are helpful in removing as much water as possible. Finally, a suction couch roll provides suction forces to improve water removal. The sheet then transfers to the pressing and drying sections for residual water removal.
The efficiency of a paper or paperboard making process is determined by many factors. At the wet end, important factors are the drainage rate of the furnish, the retention of fibers or fillers, and the formation. They affect machine runability, the production rate, white water system cleanliness, cost and quality of the product, etc. For instance, poor retention of filler and fines lowers the opacity of the paper. Ineffective single-pass retention of fiber and filler results in a high solid content in the white water system. Accumulation of fines in the headbox retards drainage. When drainage is poor the web may contain too much water to withstand the stress exerted by the press roll; therefore, the machine has to slow down and the production rate suffers. If the drainage is high, the papermakers can either pursue a higher production rate, or they can dilute the headbox furnish to allow for better formation. Formation impacts the strength, the appearance of the paper sheet and many other aspects. All these factors influence machine runability.
Improving drainage, retention, and formation are continual tasks for papermakers. The rate of water removal is often the rate determining step in the total papermaking process. For a given paper machine, the rate of water removal can be changed by changing fiber composition, degree of refining, non-fibrous materials (clay, titanium dioxide, calcium carbonate, etc.), and/or by the addition of wet end chemicals.
A wide range of retention aids, drainage aids, and formation aids are utilized in the paper industry to achieve this goal. To predict on-machine performance, the papermakers usually rely on certain types of testing devices. Unfortunately, a drainage testing device may be misleading if its drainage mechanism is dissimilar from the production machine's drainage. The effect of various drainage mechanisms will be clarified while reviewing conventional devices relevant to this invention.
Testing results obtained from most of the prior art devices are incomplete, because they do not duplicate the paper machine free water removal process. First, these devices are either for gravity or vacuum drainage exclusively. Their testing disregards the sequential drainage process normally found on paper machines, i.e., a pulsation/gravity drainage followed by a vacuum drainage. Secondly, pulsation forces are lacking in those devices. On the other hand, pulsation forces are known to have a major impact on drainage of furnishes on paper machines. Third, conventional devices cannot assess the inter-dependent variables (drainage, retention, and formation) in a single test. Details of those devices and their deficiencies are discussed in the Related Art Section. Therefore, there is a need for a device that approximates the dewatering process existing on a paper machine, is reproducible, and is laboratory-sized.